Download bop8e_chapter03 REVISED W16 Part 2

Cytoskeleton
Microtubules are cylindrical structures composed of
tubulin subunits
Microtubules and Actin Filaments
Microtubules and Actin Filaments
• Two components of the cytoskeleton—microtubules and actin
filaments—are formed from globular protein subunits
• (a) Longitudinal view and
• (b) Transverse section (made at a right angle to the long axis) of
a microtubule
• Microtubules are hollow tubules composed of two different types
of molecules, alpha (α) tubulin and beta (β) tubulin
• These tubulin molecules first come together to form soluble
dimers ("two parts") which then self-assemble into insoluble
hollow tubules
• The arrangement results in 13 "protofilaments" around a hollow
core
• (c) Actin filaments consist of two linear chains of identical
molecules coiled around one another to form a helix
Cortical Microtubules
Cortical Microtubules
• (a) A longitudinal view of cortical microtubules (indicated by
arrows) in leaf cells of the fern Botrychium virginianum
• The microtubules occur just inside the wall and plasma
membrane
• (b) A transverse view of cortical microtubules (arrows), which can
be seen to be separated from the wall by the plasma membrane
• Cortical microtubules play a role in the alignment of cellulose
microfibrils in the cell wall
Cortical
Microtubules
Botrychium
virginianum
(rattlesnake
fern)
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Widely distributed (U.S.,
Mexico, Australia, Asia,
Norway, Finland, Russia, etc.)
Used in India to treat dysentery
(an inflammation of the
intestine causing diarrhea with
blood and caused by bacteria,
viruses, parasitic worms, or
protozoa)
(image and text: Wikipedia)
Actin filaments consist of two linear chains of actin
molecules in the form of a helix
Actin Filaments
Actin Filaments
• (a) A bundle of actin filaments as revealed in an electron
micrograph of a leaf cell of maize (Zea mays)
• (b) Several bundles of actin filaments as revealed in a
fluorescence micrograph of a stem hair of tomato (Solanum
lycopersicum)
• Actin filaments are involved in a variety of activities, including
cytoplasmic streaming
Cytoplasmic Streaming in Giant Algal Cells
Cytoplasmic
Streaming in
Giant Algal
Cells
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(a) A track followed by the
streaming cytoplasm in a
giant algal cell (e.g., Chara
and Nitella)
(b) A longitudinal section
through part of the cell,
showing the arrangement
of stationary and streaming
layers of cytoplasm
The proportions have been
distorted in both diagrams
for clarity
(image: Wikipedia)
Chara globularis
Cytoplasmic
Streaming in
Giant Algal
Cells,
continued
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Chara corallina is a freshwater
plant that inhabits temperate
zone ponds and lakes
It consists of alternating nodes
and internodes
Each internodal segment is a
single large cell, up to 10 cm in
length
Because the cell is so large, it
uses cytoplasmic streaming to
distribute organelles and
nutrients throughout the
cytoplasm, which surrounds a
large central vacuole
(Johnson, Wyttenbach, Wayne,
& Hoy, 2002)
(image: Wikipedia)
A similar species, Nitella mucronata (Wikipedia)
Chara corallina cell (Johnson, et al., 2002)
Flagella and Cilia
Structure of a Flagellum
• (a) Diagram of a flagellum with its underlying basal body
• (b) Electron micrograph of the flagellum of Chlamydomonas, as
seen in transverse section
• Virtually all eukaryotic flagella have this same internal structure
• Consists of an outer cylinder of nine pairs of microtubules
surrounding two additional microtubules in the center
• The "arms," the radial spokes, and the connecting links are
formed from different types of protein
• The basal bodies from which flagella arise have nine outer
triplets, with no microtubules in the center
• The "hub" of the wheel in the basal body is not a microtubule,
although it has about the same diameter
Structure of
a Flagellum
Structure of
a Flagellum
Cell Wall
Cellulose is the principal component of plant cell walls
Primary Walls
Primary Walls
• (a) Surface view of the primary wall of a carrot (Daucus carota)
cell, prepared by a fast-freeze, deep-etch technique, showing
cellulose microfibrils, cross-linked by an intricate web of matrix
molecules
• (b) Schematic diagram showing how the cellulose microfibrils are
cross-linked into a complex network by hemicelllose molecules
• The hemicellulose molecules are linked to the surface of the
microfibrils by hydrogen bonds
• The cellulose-hemicellulose network is permeated by a network
of pectins, which are highly hydrophilic polysaccharides
• Both hemicellulose and pectin are matrix substances
• The middle lamella is a pectin-rich layer that cements together
the primary walls of adjacent cells
Primary Walls
Stone Cells
Stone Cells
• Stone cells (sclereids) from the flesh of a pear (Pyrus communis)
seen in polarized light
• Clusters of such stone cells are responsible for the gritty texture
of this fruit
• The stone cells have very thick secondary walls traversed by
numerous simple pits, which appear as lines in the walls
• The walls appear bright in polarized light because of the
crystalline properties of their principal component, cellulose
Many plant cells have a secondary wall in addition to a
primary wall
Detailed Structure of a Cell Wall
Detailed Structure of a Cell Wall
Source: Wikipedia
Detailed Structure of a Cell Wall
• (a) Portion of wall showing, from the outside in, the middle
lamella, primary wall, and three layers of secondary wall
• Cellulose, the principal component of the cell wall, exists as a
system of fibrils of different sizes
• (b) The largest fibrils, macrofibrils, can be seen with the light
microscope
• (c) With the aid of an electron microscope, the macrofibrils can
be resolved into microfibrils about 10 to 25 nanometers wide
• (d) Parts of the microfibrils, the micelles, are arranged in an
orderly fashion and impart crystalline properties to the wall
• (e) A fragment of a micelle shows parts of the chainlike cellulose
molecules in a lattice arrangement
• The middle lamella joins adjacent cells
• The primary wall is deposited while the cell is increasing in size
• The secondary wall is deposited after the primary wall has
stopped increasing in size
• Whereas the primary wall has pit-fields, the secondary wall has
pits
Primary Pit-fields, Pits, and Plasmodesmata
Primary Pit-fields, Pits, and Plasmodesmata
• (a) Cells with primary walls and primary pit-fields, which are thin
areas in the walls
• As shown here, plasmodesmata commonly traverse the wall at
the primary pit-fields
• (b) Cells with secondary walls and numerous simple pits
• (c) A simple pit-pair
• (d) A bordered pit-pair
Primary Pit-fields, Pits,
and Plasmodesmata
Primary Pit-fields, Pits,
and Plasmodesmata
Primary Pit-fields,
Pits, and
Plasmodesmata
The Layers of
Secondary Walls
The Layers of Secondary Cell Walls
• Diagram showing the organization of the cellulose microfibrils
and the three layers (S1, S2, S3) of the secondary wall
• The different orientations of the three layers strengthen the
secondary wall
Growth of the cell wall involves interactions among
plasma membrane, secretory vesicles, and microtubules
Cell Expansion
Cell Expansion
• The orientation of cellulose microfibrils within the primary wall
influences the direction of cell expansion
• (a) If the cellulose microfibrils are randomly oriented in all walls,
the cell will expand equally in all directions, tending to become
spherical in shape
• (b) If the microfibrils are oriented at right angles to the ultimate
long axis of the cell, the cell will expand longitudinally along that
axis
Plasmodesmata are cytoplasmic strands that connect
the protoplasts of adjacent cells
Synthesis of
Cellulose Microfibrils
Synthesis of Cellulose Microfibrils
• Cellulose microfibrils are synthesized by enzyme complexes that
move within the plane of the plasma membrane
• (a) The enzymes are complexes of cellulose synthase that, in
seed plants, form rosettes embedded in the plasma membrane
• Each enzyme rosette, shown here in a longitudinal section,
synthesizes cellulose from the glucose derivative UDP-glucose
(uridine diphosphate glucose)
• The UDP-glucose molecules enter the rosette on the inner
(cytoplasmic) face of the membrane, and a cellulose microfibril is
extruded rom the outer face of the membrane
• (b) As the far ends of the newly formed microfibrils become
integrated into the cell wall, the rosettes continue synthesizing
cellulose, moving along a route (arrows) that parallels the cortical
microtubules in the underlying cytoplasm
Synthesis of
Cellulose Microfibrils
Plasmodesmata
Plasmodesmata
in Diospyros
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Previous slide: Light
micrograph of plasmodesmata
in the thick primary walls of
persimmon (Diospyros)
endosperm, the nutritive tissue
within the seed
The plasmodesmata appear as
fine lines extending from cell to
cell across the walls
The middle lamella appears as
a light line between these cells
Plasmodesmata generally are
not discernible with the light
microscope, but the extreme
thickness of the persimmon
endosperm cell walls greatly
increases the length of the
plasmodesmata, making them
more visible
Images: Wikipedia
Plasmodesmata
and Desmotubules
Plasmodesmata and Desmotubules
• (a) Plasmodesmata connecting two leaf cells from the
cottonwood (Populus deltoides) tree
• (b) As seen with the electron microscope, plasmodesmata
appear as narrow, plasma-membrane-lined channels in the walls,
each traversed by a modified tubule of endoplasmic reticulum
known as the desmotubule
• Note the continuity of the endoplasmic reticulum on either side of
the wall with the desmotubules of the plasmodesmata
• The middle lamella between the adjacent primary walls is not
discernible in the electron micrographs, as is common in such
preparations
Plasmodesmata
and Desmotubules
The Cell Cycle
Virtual Cell: Mitosis
http://vcell.ndsu.nodak.edu/animations/mitosis/index.htm
Virtual Cell: Meiosis
http://vcell.ndsu.nodak.edu/animations/meiosis/index.htm
The Cell Cycle
The Cell Cycle
• Mitosis (the division of the nucleus) and cytokinesis (the division
of the cytoplasm), which together constitute the M phase, take
place after completion of the three preparatory phases (G1, S,
and G2) of interphase
• Progression of the cell cycle is mainly controlled at two
checkpoints, one at the end of G1 and the other at the end of G2
• In cells of different species or even of different tissues within the
same organism, the various phases occupy different proportions
of the total cycle
Cell Division in a Cell With a Large Vacuole
Cell Division in a Cell With a Large Vacuole
• (a) Initially, the nucleus lies along one wall of the cell, which
contains a large central vacuole
• (b) Strands of cytoplasm penetrate the vacuole, providing a
pathway for the nucleus to migrate to the center of the cell
• (c) The nucleus has reached the center of the cell and is
suspended there by numerous cytoplasmic strands
• Some of the strands have begun to merge to form the
phragmosome through which cell division will take place
• (d) The phragmosome, which forms a layer that bisects the cell,
is fully formed
• (e) When mitosis is completed, the cell will divide in the plane
occupied by the phragmosome
Mitosis and
Cytokinesis
During prophase, the chromosomes shorten and thicken
Mitosis, a Diagrammatic Representation
Mitosis, a Diagrammatic Representation
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(a) During early prophase, the four chromosomes shown here become visible as
long threads scattered throughout the nucleus
(b) As prophase continues the chromosomes shorten and thicken until each can
be seen to consist of two threads (chromatids) attached to each other at their
centromeres
(c) By late prophase, kinetochores develop on both sides of each chromosome at
the centromere
Finally, the nucleolus and nuclear envelope disappear
(d) Metaphase begins with the appearance of the spindle in the area formerly
occupied by the nucleus
During metaphase, the chromosomes migrate to the equatorial plane of the
spindle
At full metaphase (shown here), the centromeres of the chromosomes lie on that
plane
(e) Anaphase begins as the centromeres of the sister chromatids separate
The sister chromatids, now called daughter chromosomes, then move to opposite
poles at the spindle
(f) Telophase begins when the daughter chromosomes have completed their
migration
Mitosis in a Living Cell
Mitosis in a Living Cell
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Phase-contrast optics of a cell of the African blood lily (Haemanthus katherinae)
show the stages of mitosis
The spindle is barely discernible in these cells, which have been flattened to show
all of the chromosomes more clearly
(a) Late prophase: the chromosomes have condensed; A clear zone has
developed around the nucleus
(b) Late prophase – early metaphase: the nuclear envelope has disappeared, and
the ends of some of the chromosomes are protruding into the cytoplasm
(c) Metaphase: the chromosomes are arranged with their centromeres on the
equatorial plane
(d) Mid-anaphase: the sister chromatids (now called daughter chromosomes)
have separated and are moving to opposite poles of the spindle
(e) Late anaphase
(f) Telophase – cytokinesis: the daughter chromosomes have reached the
opposite poles, and the two chromosome masses have begun the formation of
two daughter nuclei; Cell plate formation is nearly complete
Dividing Cells in a Root Tip
Dividing Cells in a Root Tip
• By comparing these cells with the phases of mitosis illustrated in Figures
3—40 and 3—41, you should be able to identify the various mitotic
phases shown in this photomicrograph of an onion (Allium) root tip
Fully Condensed
Chromosome
Fully Condensed Chromosome
• The chromosomal DNA was replicated during the 5 phase of the cell
cycle
• Each chromosome now consists of two identical parts, called sister
chromatids, which are attached at the centromere, the constricted area
in the center
• The kinetochores are protein-containing structures, one on each
chromatid, associated with the centromere
• Attached to the kinetochores are microtubules that form part of the
spindle
During metaphase, the chromosomes become aligned
on the equatorial plane of the mitotic spindle
The mitotic spindle consists of a highly organized array
of kinetochore microtubules and polar microtubules
During anaphase, the sister chromatids separate and, as
daughter chromosomes, move to opposite poles of the
spindle
During telophase, the chromosomes lengthen and
become indistinct
Mitotic Spindle at Metaphase
Mitotic Spindle at Metaphase
• The spindle consists of kinetochore microtubules and overlapping polar
microtubules
• Note that the minus ends of the microtubules are at or near the poles
and the plus ends away from the poles
• Following a tug-of-war, the chromosomes have come to lie on the
equatorial plane
Cytokinesis in plants occurs by the formation of a
phragmoplast and a cell plate
Cell Plate Formation
Cell Plate Formation
• In plant cells, separation of the daughter chromosomes is followed by
formation of a cell plate, which completes the separation of the dividing
cells
• Here numerous Golgi vesicles can be seen fusing in an early stage of
cell plate formation
• The two groups of chromosomes on either side of the developing cell
plate are at telophase
• Arrows point to portions of the nuclear envelope reorganizing around the
chromosomes
Progressive Stages of Cell Formation
Progressive Stages of Cell Formation
• These electron micrographs of root cells of lettuce (Lactuca sativa) show
the association of the endoplasmic reticulum with the developing cell
plate and the origin of plasmodesmata
• (a) A relatively early stage of cell plate formation, with numerous small,
fusing Golgi vesicles and loosely arranged elements of tubular (smooth)
endoplasmic reticulum
• (b) An advanced stage of cell plate formation, revealing a persistent
close relationship between the endoplasmic reticulum and fusing
vesicles
• Strands of tubular endoplasmic reticulum become trapped during cell
plate consolidation
• (c) Mature plasmodesmata, which consist of a plasma-membrane-lined
channel and a tubule, the desmotubule, of endoplasmic reticulum
Progressive Stages of Cell Formation
Progressive Stages of Cell Formation
Fluorescence Micrographs of Microtubular Arrays
in Root Tip Cells of Onion (Allium cepa)
Fluorescence Micrographs of Microtubular
Arrays in Root Tip Cells of Onion (Allium cepa)
• (a) Prior to formation of the preprophase band of microtubules, most of
the microtubules lie just beneath the plasma membrane
• (b) A preprophase band of microtubules (arrowheads) encircles the
nucleus at the site of the future cell plate
• Other microtubules (arrows), forming the prophase spindle, outline the
nuclear envelope, which itself is not visible
• The lower right-had cell is at a later stage than that above
• (c) The mitotic spindle at metaphase
• (d) During telophase, new microtubules form a phragmoplast, in which
cell plate formation takes place
Microtubule Arrays and the Cell Cycle
Microtubule Arrays and the Cell Cycle
• Changes in the distribution of microtubules during the cell cycle and cell
wall formation during cytokinesis
• (a) During interphase, and in enlarging and differentiating cells, the
microtubules lie just inside the plasma membrane
• (b) Just before prophase, a ringlike band of microtubules, the
preprophase band, encircles the nucleus in a plane corresponding to the
equatorial plane of the future mitotic spindle, and microtubules of the
prophase spindle begin to assemble on opposite sides of the nucleus
• (c) During metaphase, the microtubules form the mitotic spindle
• (d) During telophase, microtubules are organized into a phragmoplast
between the two daughter nuclei
• The cell plate, made up of fusing Golgi vesicles guided into position by
the phragmoplast microtubules, forms at the equator of the
phragmoplast
• (e) As the cell plate matures in the center of the phragmoplast, the
phragmoplast and developing cell plate grow outward until they reach
the wall of the dividing cell
Microtubule Arrays and the Cell Cycle,
continued
• (f) During early interphase, microtubules radiate outward from the
nuclear envelope into the cytoplasm
• (g) Each sister cell forms its own primary wall
• (h) With enlargement of the daughter cells (only the upper one is shown
here), the mother cell wall is torn
• In (g) and (h) the microtubules once more lie just inside the plasma
membrane, where they play a role in the orientation of newly forming
cellulosic microfibrils
Summary of Main Points of this Chapter
• The cell is the fundamental unit of life
• Cells are of two fundamentally different types: prokaryotic and
eukaryotic
• Plant cells typically consist of a cell wall and a protoplast
• The nucleus is surrounded by a nuclear envelope and contains
nucleoplasm, chromatin, and one or more nucleoli
• Ribosomes are the sites of protein synthesis
• There are three main types of plastids: chloroplasts,
chromoplasts, and leucoplasts
• Mitochondria are the sites of respiration
• Plastids and mitochondria share certain features with prokaryotic
cells
• Peroxisomes are surrounded by a single membrane
Summary of Main Points of this Chapter,
continued
• Vacuoles perform a variety of functions
• The endoplasmic reticulum is an extensive three-dimensional
system of membranes with a variety of roles
• The Golgi apparatus is a highly polarized membrane system
involved in secretion
• The cytoskeleton is composed of microtubules and actin
filaments
• The cell wall is the major distinguishing feature of the plant cell
• Dividing eukaryotic cells pass through a regular sequence of
events known as the cell cycle
• During prophase, the duplicated chromosomes shorten and
thicken
• Metaphase, anaphase, and telophase followed by cytokinesis
result in two daughter cells
References
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Heinze, M., Reichelt, R., Kleff, S., & Eising, R. (2000). High resolution scanning
electron microscopy of protein inclusions (cores) purified from peroxisomes of
sunflower (Helianthus annuus L.) cotyledons. Crystal Research and Technology,
35(6-7). http://dx.doi.org/10.1002/1521-4079(200007)35:6/7<877::AIDCRAT877>3.0.CO;2-S
Hu, J., Baker, A., Bartel, B., Linka, N., Mullen, R. T., Reumann, S., & Zolman, B.
K. (2012). Plant peroxisomes: Biogenesis and function. The Plant Cell, 24(6),
2279-2303. http:/?/?dx.?doi.?org/?10.?1105/?tpc.?112.?096586
Also:
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Wikipedia, as indicated throughout